Fly ash microsphere (FAM), a high value-added by-product, are powdered hollow beads separated from fly ash in power plants. This paper investigated the effects of FAM as a mineral admixture on the 3 d hydration and microstructure of cement. A composite binder containing inert quartz powder with a surface area similar to that of FAM was selected as the control sample. The results show that FAM has a low reaction degree at room temperature because FAM mainly exerts physical effects, and almost no pozzolanic reactions occur. However, increasing the curing temperature can significantly activate the pozzolanic activity of FAM and increase the reaction degree by up to 20% at 3 d. The reaction of FAM reduces the macropore volume, leading to a denser microstructure due to the pozzolanic reaction. Therefore, increasing the curing temperatures decreases the gap of the compressive strength between the paste containing FAM and the cement paste. (C) 2020 Elsevier B.V. All rights reserved.